I cannot get solder to stick to the PCB. I've scraped off a 2mm strip, heated it, added flux with pen, reheated, added rosin solder (lead-free) and it won't stick. It will occasionally form ball-like blobs that look attached but come apart under minimal force. Solder sticks to the metal cage of the USB port fine. I've never been very good at soldering but I'm starting to think it's my tools more than me. Any advice?